1. Field of the Invention
The present invention relates to a signal processing apparatus for concealing uncorrectable errors during signal reproduction in a digital signal recording and reproducing apparatus for digitizing a video signal and recording and reproducing the digitized signal.
2. Description of the Prior Art
Digital VCRs, which record and reproduce a digitized video signal to achieve significantly better image quality in consumer-class video devices, are widely expected to replace today's analog VCRs as the next generation in video components. One of the greatest advantages of the digital VCR is the nearly total absence of signal deterioration through multiple generations of dubbing and editing, and digital VCRs are already used in broadcasting and other commercial applications. To achieve a long playing time in a compact cassette, however, requires the efficient reduction of the image data by means of bit rate reduction processing. The structure and operation of a digital VCR using bit rate reduction is described briefly below.
The first step during recording is to convert the input video signal to a digital signal using an analog/digital (A/D) converter, after which the signal is compressed to a predetermined data quantity by a bit rate reduction encoder. The discrete cosine transformation (DCT) method is widely used for bit rate reduction or video signals because of the compression efficiency of the method. In the DCT method plural pixels are grouped into blocks, and the blocks are then converted to a frequency domain. The data can be efficiently compressed by variable length coding according to the converted coefficient. The encoding efficiency can be increased in this case by increasing the length of the variable length coding units. Doing so, however, also increases the range of error propagation when an error occurs. A coding method whereby variable length coding is applied with plural block units ("compression blocks" below) has been proposed in Japanese patent application number 1989-147891 for use in a digital VCR with a higher error probability than other transmission paths.
Error correction parity is then added by the error correction encoder, and a synchronization signal and identification data (ID) are added to create recording blocks ("sync blocks" below). The signal is then modulated for recording and the data is recorded to tape.
These sync blocks include the synchronization signal "sync" for detecting the sync blocks from the reproduced signal, the track number for correctly writing the reproduced sync block to memory, the sync block number and/or other identification information (ID), the video signal in a bit rate reduction form, and error correction parity (inner parity) data. These sync blocks are then aligned with the track and recorded to tape.
During signal reproduction, the sync blocks are detected from the signal reproduced by the playback head and demodulated, and error correction is applied by the error correction decoder based on the error correction parity added during recording. Any uncorrectable errors are then concealed in the bit rate reduced state by the error concealment processor.
The error correction process can correct up to a certain number of errors. When the tape is scratched or otherwise damaged, resulting in significant signal dropout, however, the amount of errors exceeds the correction capability of the process, and error correction becomes impossible. When such uncorrectable errors occur, concealment is used so that the effect of the errors is not visually conspicuous. Because the effect of these errors is propagated through the completed range of variable length coding, concealment is achieved by the error concealment processor substituting a previous corresponding sync block for the complete sync block containing the compression block in which the error is found. The concealed data is then decoded to the original data by the bit rate reduction decoder, converted to an analog signal by the D/A converter, and output.
The following problems are inherent in the above process and apparatus.
In digital VCRs that do not use bit rate reduction for broadcasting and other commercial applications, errors are concealed by interpolation of the error pixels from the surrounding pixels, or by substituting the corresponding pixel from the previous field. When bit rate reduction by an orthogonal transform or other block unit is used, a single error affects the complete block, and it is therefore difficult to conceal errors by interpolation from surrounding pixel level data. Concealment must therefore rely on substituting a complete block from the preceding field.
Such substitution successfully conceals errors without the viewer noticing the concealed blocks in still image areas, but the loss of correlation between fields in image areas with much movement results in a loss of continuity between the concealed block and the surrounding blocks. The viewer perceives this as noticeable image deterioration. When bit rate reduction is applied in plural field units to increase the coding efficiency, deterioration caused by concealment becomes an even greater problem because the concealed block is replaced by a block from several fields earlier.